1. Field of the Invention
This invention relates to a subtraction processing method for radiation images and an apparatus for carrying out the method. More particularly to a subtraction processing method in a radiation image recording and reproducing method comprising the steps of exposing the stimulable phosphor sheet carrying a radiation image stored therein to stimulating rays to sequentially release the radiation energy stored in the stimulable phosphor sheet as light emission, photoelectrically reading out the emitted light by use of a photodetector, and reproducing the obtained image signal as a visible image, and an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and read-out system. Specifically, the stimulable phosphor formed on a sheet is first exposed to a radiation transmitting through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as laser beam which causes it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as photographic light-sensitive material or on a display such as cathode ray tube (CRT).
This radiation image system using the stimulable phosphor sheet is advantageous over the conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desirable density regardless of the amount of exposure of the phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium or a display. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly diagnostic purposes. This is very advantageous in practical use.
As mentioned above, in the radiation image system using a stimulable phosphor, deviation of the level of the radiation energy stored in the stimulable phosphor from a desired level can easily be compensated by setting the read-out gain to an appropriate value when photoelectrically reading out the light emitted from the stimulable phosphor upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a change in radiation dose due to fluctuating tube voltage or MAS value of the radiation source, a variation in the sensitivity of the stimulable phosphor or the photodetector, a change in radiation dose according to the condition of the object, or a change in the radiation transmittance according to the object etc. Further, it is possible to obtain a desirable radiation image even when the radiation dose to the object is reduced. Further, it is possible to obtain a radiation image having high image quality of high contrast, high sharpness and low noise etc. by once converting the light emitted from the stimulable phosphor into an electric signal, and processing the electric signal as desired. Particularly, when the radiation image is used for medical diagnosis, it is possible to obtain a radiation image processed in accordance with a desired portion of a human body such as the heart, the chest etc. and improve the diagnostic efficiency and accuracy.
There has heretofore been known a subtraction processing method wherein an image of a specific structure (for example, an organ, the bone, the blood vessel, or the like) of human body is extracted by use of two or more X-ray photographic films to correctly diagnose the specific structure. In general, the known subtraction processing method is classified into the so-called temporal (time difference) subtraction processing method and the so-called energy subtraction processing method. In the former method, an image of a specific structure is extracted by subtracting the digital image signal of an X-ray image obtained without injection of contrast media from the digital image signal of an X-ray image in which the image of the specific structure is enhanced by the injection of contrast media. In the latter method, an object is exposed to X-rays having energy distributions different from each other to obtain two X-ray images respectively containing the images of a specific structure recorded on the basis of the intrinsic X-ray energy absorption characteristics of the specific structure. Then, each X-ray image is weighted appropriately, and subjected to subtraction to extract the image of the specific structure.
However, the aforesaid subtraction processing method using the X-ray photographic films is disadvantageous in that, since the X-ray photographic films generally exhibit non-linear gradation and a narrow latitude of exposure, it is impossible to obtain a subtraction image of high quality. Further, in this conventional subtraction processing method using the X-ray photographic films, one X-ray image is inverted, two X-ray photographic films are manually superposed one upon the other, and the subtracted image is recorded on a third photographic film. Therefore, it is not always possible to correctly superpose the X-ray images recorded on the two X-ray photographic films one upon the other and remove images other than the structure to be diagnosed, and it is very troublesome to match the positions of the X-ray photographic films to each other. Accordingly, the aforesaid subtraction processing method using the X-ray photographic films is not always effective for diagnosis, and has not been used widely, except for a particular use.
Recently, the so-called digital subtraction processing method or digital radiography (hereinafter referred to as "DR") has attracted attention since, if the image data is a digital value, subtraction can be conducted by a linear computer processing without using the troublesome, non-linear photographic subtraction technique. As the DR, there have heretofore been known digital fluoroscopy wherein the output of an X-ray fluoroscopic camera comprising an image intensifier tube (I.I. tube) and a television camera is digitally processed, and scanned projection radiography utilizing the X-ray detecting system of computed tomography, such as Xe detector. The subtraction image obtained by DR is particularly advantageous over the subtraction image obtained by use of the conventional X-ray photographic films in that subtraction can be electrically conducted by digital processing. However, the DR presents a problem that the spatial resolution of the subtraction image obtained by use of the DR generally depends on the resolution of the X-ray image detector such as I.I. tube, Xe detector, or the like, and that the spatial resolution of the subtraction image becomes lower than that obtained by the conventional method using the X-ray photographic films, making it impossible to sufficiently accurately diagnose a specific structure. Further, since the recording range in the DR is limited by the light receiving area of the X-ray image detector such as I.I. tube, Xe detector, or the like, the DR presents another problem that it is impossible to obtain a subtraction image of a wide range of portion of the human body at one time.
Also in the aforesaid radiation image system using a stimulable phosphor sheet, it is possible to conduct various digital processings since a final radiation image can be reproduced on various output devices after reading out a radiation image once stored in the stimulable phosphor sheet by use of stimulating rays, detecting the light emitted from the stimulable phosphor sheet upon stimulation thereof by a photodetector, converting the thus detected electric signal into a digital signal, and processing the signal in various ways. Namely, if said radiation image system utilized for the subtraction processing, it is possible to obtain the advantage of the aforesaid DR, i.e. the advantage that it is possible to conduct a digital processing. Further, said radiation image system can provide an image having markedly higher spatial resolution compared with the conventional DR since it is possible to decrease the beam diameter of the stimulating rays (laser beam) employed for scanning the stimulable phosphor sheet, increase the number of picture elements per unit area, and directly record the final output of the image data obtained by the subtraction processing and various image processings on a light-sensitive material such as silver halide photographic material. Therefore, theoretically, it is possible to obtain a sharp subtraction image having a spatial resolution higher than visual resolution of human eyes. Further, since there is no technical obstruction to make and use the larger size of the stimulable phosphor sheet, it is possible to obtain at one time a subtraction image over a large area covering a wide range of portion of the human body. Thus, the radiation image system using a stimulable phosphor sheet has many important features that the conventional DR does not possess.
However, experiments conducted to obtain a subtraction image in the aforesaid radiation image system using a stimulable phosphor sheet revealed the problems described below.
Namely, when a subtraction image is obtained in the radiation image system using a stimulable phosphor sheet, two stimulable phosphor sheets (in some cases, three or more stimulable phosphor sheets) are sequentially or simultaneously inserted into an image recording table, radiation images to by subtraction processed are recorded on the stimulable phosphor sheets, the stimulable phosphor sheets carrying the radiation images stored therein are then inserted one by one into an image read-out apparatus, and the radiation images to be subtraction processed are read out from the stimulable phosphor sheets. During this course, even when the recording and the read-out are conducted very carefully, a shift and a rotation occur between the images to be subtraction processed. As a result, an image to be erased in the subtraction processing is not erased, or an image to be extracted is erased to develop an artifact. In this case, therefore, a correct subtraction image cannot be obtained, and a very real problem is presented with respect to diagnosis.
In the radiation image system using a stimulable phosphor sheet, a radiation image is stored as a latent image in the stimulable phosphor sheet. Therefore, when a deviation occurs between the radiation images stored in the stimulable phosphor sheets, the two X-ray images connot be visually matched to each other, and it is not always possible to correct the deviation, differently from the case of X-ray photographic films on which X-ray images are recorded as visible images.
Further, even when the shift and the rotation between the two radiation images can be detected by use of some means, much time is required for the conventional operation processing to be conducted to correct the detected data of the radiation images, particularly in the case of correction of the rotation. This is a very real problem in practical use.
To solve the above problem, it has been proposed in European patent application No. 83 102787.5 to employ a subtraction processing method for radiation images comprising the steps of (i) when each radiation image to be subtraction processed is recorded on each stimulable phosphor sheet, simultaneously recording a marker for providing a reference point or a reference line to said stimulable phosphor sheet, (ii) detecting the spatial coordinate of said reference point or said reference line from the digital image signals of said image detected from said stimulable phosphor sheet, (iii) conducting the aforesaid steps for two or more radiation images to be subtraction processed, (iv) calculating the rotation and the shift among said two or more radiation images based on the detected spatial coordinates, (v) digitally rotating and/or moving either one of said radiation images to be subtraction processed based on the calculated rotation and the calculated shift,and, then conducting a subtraction processing of the image signals among the corresponding picture elements of said two or more radiation images. The term "shift" as used herein means a longitudinal deviation or a transverse deviation of the radiation image or an object with respect to the stimulable phosphor sheet.
The aforesaid subtraction processing method can automatically correct the shift and rotation occurring among the radiation images stored in the stimulable phosphor sheets. Accordingly, this method can provide a subtraction image which exhibits high contrast resolution and high spatial resolution and which is free of any artifact and very suitable for viewing, particularly for diagnostic purposes.
Further, when combined with the approximate rotation operation as described in European patent application No. 83 102787.5, the aforesaid subtraction processing method can correct a deviation in position very quickly compared with the conventional operation processing method.
However, in the aforesaid subtraction processing method, it takes much time (several minutes to several tens of minutes) even for a large computer to conduct the operation for correcting a deviation in position among the radiation images to be subtraction processed even when the approximate rotation operation is employed in combination therewith. Even when a special operation processing unit for correcting a deviation in position is employed, it takes one to three minutes for the operation processing unit to conduct the operation. In order to further increase the operation speed, it is necessary to employ a more roughly approximated rotation operation. In this case, the image quality of the subtraction image is deteriorated compared with the image quality of the subtraction image obtained by using a strict rotation operation. The aforesaid method is practical only when a large computer or a special-purpose. operation processing unit exhibiting high operation speeds. In order to obtain a subtraction image by this method, it is necessary to use a very expensive large computer or a special-purpose operation processing unit having a limited operation function and low flexibility in application. Further, even when such a large computer or a special-purpose operation processing unit is used, it takes a period of one minute or more for the operation to be conducted.